A bone marrow aspirate evaluation should always be performed by a clinical pathologist. However, the components of bone marrow evaluation will be described here to aid your understanding of how to interpret changes in the bone marrow. Bone marrow evaluation includes assessments of bone marrow cellularity, hematopoietic precursors (including an M:E ratio), iron stores, and other cell types.
Cellularity of marrow is best assessed on core biopsy, but can usually be estimated on aspirated samples as long as there are plenty of bone marrow particles in the sample. If there are only a few bone marrow particles on the slide it may indicate either inadequate sampling or the presence of a disease that can lead to poorly cellular bone marrow samples (e.g. myelofibrosis, or marrow necrosis). Evaluation of cellularity aids in determination of whether cell lines are increased or decreased within the marrow. Only nucleated cells are included in assessment of cellularity (not mature RBCs or reticulocytes, which could just indicate blood in the sample).
All three lineages of hematopoietic precursors are evaluated: erythroid, myeloid , and megakaryocytic. The majority of the precursors are erythroid and myeloid, with fewer megakaryocytes. The numbers of megakaryocytes are assessed subjectively, and normal numbers depend on the number and size of the particles present. Megakaryocyte numbers are best assessed on core biopsy, but can usually be estimated on aspirates.
The majority of nucleated cells in the bone marrow are of erythroid and myeloid lineages. These should mostly be the more mature forms, i.e. metarubricytes and segmented neutrophils. Less mature forms should be present in progressively decreasing numbers. As part of the assessment of erythroid and myeloid lineages, we calculate a myeloid to erythroid ratio (M:E ratio). The M:E ratio indicates the relative numbers of myeloid lineage cells (granulocytic and monocytic) to nucleated erythroid precursors in marrow. This ratio is interpreted in relation to peripheral blood CBC results and bone marrow cellularity to help identify myeloid hyperplasia or hypoplasia and erythroid hyperplasia or hypoplasia. In most species, the M:E ratio is normally about 1:1 to 2:1.
For example, if the M:E ratio is increased, it indicates that there is either myeloid hyperplasia (an increase in the “M” portion) and/or erythroid hypoplasia (a decrease in the “E” portion). In order to determine which (or both) is present, you have to look at the bone marrow cellularity and the CBC results. If the animal has an increased bone marrow cellularity and an inflammatory leukogram on the CBC, you would interpret the increased M:E ratio to be due to myeloid hyperplasia. If the animal has decreased bone marrow cellularity and a non-regenerative anemia, you would interpret the increased M:E ratio to be due to erythroid hypoplasia. To add a layer of complication, you can have multiple processes occurring simultaneously. For example, an animal with an inflammatory disease will likely have myeloid hyperplasia contributing to an increase in the M:E ratio, and will likely also have anemia of inflammatory disease in which there is suppression of erythropoiesis leading to erythroid hypoplasia which also contributes to the increase in the M:E ratio.
Stainable iron stores are normally visible in most species except cats. It is seen as hemosiderin within macrophages, which appears as a golden-brown pigment with Wright’s stain. Iron can be seen more distinctively using Prussian blue stain (an iron-specific stain).
Evaluation of other cell types
We evaluate other cell types that can normally be present in low numbers in the bone marrow (i.e. lymphocytes, plasma cells, mast cells, and macrophages). Any abnormal cell types (e.g. leukemia or infiltrating neoplasia) or infectious organisms (e.g. Histoplasma, Leishmania, etc.) are also reported.